1. Field of the Invention
The present invention relates generally to heat transfer systems for co-generation units; and, more particularly, to heat transfer and cooling systems for internal combustion engine driven co-generation units.
2. Description of Related Art
Electric energy generation in this country has lagged behind demand. There are a number of reasons for this, but chief among them is failure of traditional energy producers to replace spent units and capitalize new plants. This has been, in part, due to increased air quality regulations. In addition new challenges face electric generation-security. Events of Sep. 11, 2001 showed this nation its vulnerability to terrorist attack. Vital operations, such as police, medical and civil defense that relied upon the electric power xe2x80x9cgridxe2x80x9d for service, realized that their needs were susceptible to disruption and viewed stand-alone units as well as micro grids as a possible solution. These alternatives are fraught with their own problems. Chief among the reasons is a drastic increase in demand. Thus, while energy demand has increased, generating capabilities have not.
One reason for the growth in demand is the increased use of computers and other technology for industrial and business purposes, as well as personal use. As computer usage continues to grow, the use of power-consuming peripheral technologies, such as printers, cameras, copiers, photo processors, servers, and the like, keep pace and even expand. As business use of computer based equipment continues to rise, as do the number of in-house data servers, outsourced data storage facilities, financial systems, and Internet-related companies requiring constant electrical uptime and somewhat reducing traditional peak demand times, requirement for reliable, cheap, environmentally compliant electrical power continues to grow.
Other technological advances have also increased electrical energy demand. Increased use of power consuming devices in every aspect of life from medical to industrial manufacturing robots, as well as innovations in almost every research and industrial field are supported by increasingly complex technology, which requires more electrical power to function. CAT scans, NMRs, side looking X-rays, MRIs and the like all take electrical power.
As a result, the Federal Government deregulated power generation, and a number of states have begun to establish competitive retail energy markets. Unfortunately, the deregulation process has not provided adequate incentives for industry entities to construct generating facilities, upgrade the transmission grid, or provide consumers with price signals to enable intelligent demand-side management of energy consumption. With the deregulation in the utility market, energy (kWh) has become a commodity item that can be bought or sold. However, swings in supply and demand leave end users open to fluctuations in the cost of electricity.
According to the ETA, to meet projected increases in demand over the next 20 years, at least 393 GW of additional generating capacity must be added. In some areas, the growth in demand is much higher than the projected two percent average (e.g., California""s peak electricity demand grew by 18 percent between 1993 and 1999, while generating capacity increased by only 0.3 percent.) Despite California""s highly publicized energy situation, a similar problem exists for other states as well; the New York Independent System Operator recently stated that 8600 MW of additional generating capacity (a 25 percent increase) must be added by 2005 to avoid widespread shortages that may lead to blackouts.
In addition to the mismatch between demand and generating capacity, the physical transmission infrastructure necessary to deliver power from geographically remote generating facilities to the consumer""s location is unable to support the increased load. Even under today""s operating conditions, the transmission grid is subject to stress and occasional failure.
Additionally, security and reliability of source has become of increasing concern. Vulnerability of grid systems and blackouts have become more commonplace. Strategic industries are looking to cut energy costs, increase reliability, and assure security. This has lead to an interest in distributed market technologies. The potential market for distributed generation has become vast without adequate means for fulfilling this need. Again, inefficiency, reliability, and environmental concerns are major barriers. The compelling economics are made on engine efficiency without the financial benefit of waste heat usage, yet with all of the same customer reluctance to accept hassles. Industry estimates indicate that the existing market for distributed generation is $300 billion in the United States and $800 billion worldwide.
The need to leverage existing technology while transitioning to alternative energy sources is an important driver for meeting this challenge. Although most existing distributed generation sites use small gas turbine or reciprocating engines for generation, there are many alternatives that are being considered over the longer term. Technologies, such as micro turbines, are currently available, but only used at a relatively small number of sites. These newer generators offer some inherent advantages, including built-in communications capabilities. It is anticipated that fuel cells will be available in the next five years, which will provide some highly appealing, environmentally friendly options.
As it stands today however, small gas turbine and reciprocating engines comprise a substantial proportion of existing generator technology in the market and will for some time to come for a number of reasons. Engines provide the best conversion efficiency (40%), and they can operate using non-pressurized gas. Micro turbines, on the other hand, require compressed gas and conversion efficiency is lower (approximately 30%). These latter generators tend to be used in wastewater and landfill and other specialty sites, where a conventional prime mover is unable to stand up to poor fuel quality. Therefore, for utilities to truly benefit from a distributed generation scheme over the short term, they must look to the existing generator technology to provide a sustainable and affordable solution.
Waste heat utilization or co-generation is one way to meet this challenge. In the case of power generation, the waste heat is not used, and the economics are based largely on the cost of the electricity produced (i.e. heat rate is paramount), with little consideration for improved reliability or independence from the electric grid. The anticipated fluctuation in energy costs, reduced reliability, and increasing demand has led end users to consider maximizing efficiency through use of heat from generation of on-site generating-heat capture systems, i.e. co-generation, or xe2x80x9cCombined Heating and Powerxe2x80x9d (CHP).
Co-generation of electricity and client process/utility service heat to provide space heating and/or hot water from the same unit is one solution. Co-generation provides both electricity and usable process or utility heat from the formerly wasted energy inherent in the electricity generating process. With co-generation, two problems are solved for the price of one. In either case, the electricity generation must meet stringent local air quality standards, which are typically much tougher than EPA (nation wide) standards.
On-site co-generation represents a potentially valuable resource for utilities by way of distributed generation. A utility can increase capacity by turning to a xe2x80x9chostxe2x80x9d site (e.g. industrial user) with an existing generator, and allow them to parallel with the grid and use their generator capacity to handle peak volumes. From the utility""s point of view, the key advantages to a distributed generation solution are twofold: improved system reliability and quality; and the ability to defer capital costs for a new transformer station.
For customers who can use the process/utility waste heat, the economics of co-generation are compelling. The impediment to widespread use is reliability, convenience, and trouble-free operation. Co-generation products empower industrial and commercial entities to provide their own energy supply, thus meeting their demand requirements without relying on an increasingly inadequate public supply and infrastructure.
Unfortunately, to date, the most widespread and cost-effective technologies for producing distributed generation and heat require burning hydrocarbon-based fuel. Other generating technologies are in use, including nuclear and hydroelectric energy, as well as alternative technologies such as solar, wind, and geothermal energy. However, burning hydrocarbon-based fuel remains the primary method of producing electricity. Unfortunately, the emissions associated with burning hydrocarbon fuels are generally considered damaging to the environment, and the Environmental Protection Agency has consistently tightened emissions standards for new power plants. Green house gases, as well as entrained and other combustion product pollutants, are environmental challenges faced by hydrocarbon-based units.
Of the fossil fuels, natural gas is the least environmentally harmful. Most natural gas is primarily composed of methane and combinations of Carbon Dioxide, Nitrogen, Ethane, Propane, Iso-Butane, N-Butane, Iso-Pentane, N-Pentane, and Hexanes Plus. Natural gas has an extremely high octane number, approximately 130, thus allowing higher compression ratios and broad flammability limits. A problem with using natural gas is reduced power output when compared to gasoline, due mostly to the loss in volumetric efficiency with gaseous fuels. Another problem area is the emissions produced by these natural gas engines. Although, the emissions are potentially less than that of gasoline engines, these engines generally require some types of emissions controls such as exhaust gas re-circulation (EGR), positive crankcase ventilation (PCV), and/or unique three-way catalyst. A still another problem with using natural gas is the slow flame speed, which requires that the fuel be ignited substantially before top dead center (BTDC). In general, most internal combustion engines, running on gasoline, operate with a spark advance of approximately 35xc2x0 F. BTDC; where as, the same engine operating on natural gas will require an approximate advance of 50xc2x0 F. BTDC. The slower burn rate of the fuel results in reduced thermal efficiency and poor burns characteristics. Never the less natural gas fueled engines provide a valuable power source for distributed generation.
Internal combustion engines utilized for combined heat and power are designed so that engine coolant from the radiator passes through a process/utility heat exchanger so the heat from combustion can be transferred to a co-generation client. Prior art co-generation systems employing internal combustion engines, and specifically, natural gas fueled engines have suffered from the myriad of problems including elevated head temperatures and inability to deliver large quantities of process and/or utility heat to the co-generation client. Excessive head temperatures lead to inefficient operation and unacceptable environmental conditions, which include excessive use of fuel as well as significant thermal NOx production.
It is well known that emission reduction for natural gas engines can be accomplished by recycling of exhaust gases to make the engines xe2x80x9crun lean.xe2x80x9d Numerous systems have been devised to recycle exhaust gas into the fuel-air induction system of an internal combustion engine for the purposes of pre-heating the air-fuel mixture to facilitate its complete combustion in the combustion zone, for re-using the unignited or partially burned portions of the fuel which would otherwise pass to exhaust and into the atmosphere, and for reducing the oxides of nitrogen emitted from the exhaust system into the atmosphere. It has been found that approximately 15 to 20 percent exhaust gas recycling is required at moderate engine loads to substantially reduce the nitrogen oxide content of the exhaust gases discharged in the atmosphere, that is, to below about 1,000 parts per million.
Although the prior art systems have had the desired effect of reducing nitrogen oxides in the exhaust by reducing the maximum combustion temperature as a consequence of diluting the fuel-air mixture with recycled exhaust gases during certain operating conditions of the engine, these systems have not been commercially acceptable from the standpoints of both cost and operating efficiency and have been complicated by the accumulation of gummy deposits which tend to clog the restricted bypass conduit provided for recycling the exhaust, and have also been complicated by the desirability of reducing the recycling during conditions of both engine idling when nitrogen oxide emission is a minor problem and wide open throttle when maximum power is required, while progressively increasing the recycling of exhaust gases with increasing engine load at part open throttle.
The nitrogen oxide emission is a direct function of combustion temperature, and for that reason is less critical during engine idling when the rate of fuel combustion and the consequent combustion temperature are minimal but tends to be problematic during throttle up and extended full speed operation. In the usual hydrocarbon fuel type engine, fuel combustion can take place at about 1,200xc2x0 F. The formation of nitrogen oxides does not become particularly objectionable until the combustion temperature exceeds about 2,200xc2x0 F., but the usual engine combustion temperature, which increases with engine load or the rate of acceleration at any given speed frequently, rises to about 2,500xc2x0 F. It is known that the recycling of at least one-twentieth and not more than one-fourth of the total exhaust gases through the engine, depending on the load or power demand, will reduce the combustion temperature to less than 2,200xc2x0 F. Contaminants in the exhaust resulting from fuel additives desired for improved combustion characteristics normally exist in a gaseous state at combustion temperatures exceeding about 1,700xc2x0 F., but tend to condense and leave a gummy residue that is particularly objectionable at the location of metering orifices and valve seats in the exhaust recycling or bypass conduit.
Thus, natural gas fired internal combustion driven co-generation systems have previously suffered from one or more disadvantages. Specifically, the EGR system did not recycle exhaust gas to the intake engine manifold at sufficiently low temperature to foster low cylinder head temperatures. Simultaneously, turbo charged fuel systems, because of the compression, increased intake fuel manifold temperatures causing the same affect. Additionally, engine-cooling systems were not efficient enough to remove substantial engine heat from the cooling fluid while maintaining an inlet temperature of the coolant sufficient to reduce head temperatures to an acceptable level. This in turn reduced the heat, which was transferred to the co-generation client. However, increasing coolant flow through the engine increases parasitic load decreasing efficiency. The result was a rich burning engine, i.e. inefficient, with substantial thermal NOx production, violating air emission standards, while not providing sufficient heat transfer to the process/heat co-generation loop to be worthwhile.
A further drawback was that recycling exhaust gas increased the intake air temperature and, therefore, increased the head temperature. This is particularly true when the inlet gas is supercharged. This combination of disadvantages made natural gas fueled, internal combustion driven co-generation systems an unacceptable candidate for client based distributed generation complexes.
It would be, therefore, advantageous to have a system, which reduced fuel consumption, as well as NOx production while delivering substantial heat to the process/utility heat co-generation system. In addition, it would be advantageous to run a lean burning engine using recycled exhaust gas, which results in not only a lean burn but also reduced head temperatures leading to reduces thermal emissions and greater efficiency.
It has now been unexpectedly discovered that a system for engine cooling and effective heat transfer to a co-generation client, reduces engine head temperature thereby reducing fuel consumption and reducing pollutants, as well as delivering substantially increased heat to a co-generation process/utility heat facility. The cooling cycles and process/utility heat radiation configurations of the inventive system maintain cylinder inlet temperature resulting in improved efficiency, reduced thermal NOx and longer engine life. This allows operation of the engine at optimum inlet and outlet temperatures regardless of co-generation process/utility heat system requirements, without excessive parasitic pump loads.
In accordance with the invention, a split flow engine cooling system includes a first coolant loop which directs coolant through the engine block, and a second loop which directs coolant through the at least one exhaust manifold in cooperation with the first loop, such that the coolant inlet temperature of the first loop is substantially reduced to maintain appropriate engine head temperatures to reduce thermal NOx while maintaining efficiency. The two loops then merge at a process heat exchanger such that the combined output heat contained in the liquid of the two loops is effective to deliver increased heat to the co-generation process/utility heat system without an increase in parasitic load, i.e. using the engine internal pump only.
Advantageously, the coolant loops each carry different quantities of coolant to assure engine performance. In one embodiment, the loops can be balanced by means of a dynamic feed back valving to assure head temperatures within a specified range.
In accordance with another aspect of the instant invention, a turbo intercooler heat exchanger is used to reduce the temperature of compressed engine intake gas, emerging from the turbocharger, prior to its entry into the intake manifold of the engine such that the inlet gas temperature is reduced to retard the formation of thermal NOx. Thus the engine driven coolant pump can be utilized exclusively for the coolant loop, reducing the parasitic load, while drastically reducing cylinder inlet temperature resulting in improved efficiency, lower thermal NOx and longer engine life.
In another aspect an EGR cooling circuit using air finned heat exchangers is used to reduce the temperature of the recycled exhaust gas, prior to its mixing with the intake gases for combustion. This further reduces cylinder inlet temperature resulting in improved efficiency, lower thermal NOx, and longer engine life.
In accordance with the invention a dump/balance radiator is used to remove heat not transferred to the co-generation process/utility heat system such that engine efficiency is maintained even in the absence of the co-generation process/utility heat system load.